1. Field of the Invention
The present invention relates to a method of forming insulation films, e.g., for a liquid crystal display, and more particularly to a method of forming insulation films for a liquid crystal display which is adapted to form insulating films without using an expensive apparatus or complicated processes.
2. Description of the Related Art
Generally, amorphous silicon or low temperature polycrystalline silicon is used to form thin film transistors for a liquid crystal display. Polycrystalline silicon has a higher mobility of carriers such as electrons or holes than that of amorphous silicon. Thin film transistors made from the polysilicon can be mounted on a panel without additional driving circuitry. Thus, low temperature polysilicon is widely used for thin film transistors.
The fabrication of thin film transistors using low temperature polycrystalline involves the step of crystallizing amorphous silicon using means such as a laser. This fabrication process also involves forming gate insulating films between gate electrodes and active layers (which were previously obtained by the crystallizing method using the laser), and forming insulating films (such as passivation films used as protection layers covering source/drain electrodes), where the insulating films are composed of silicon oxides or silicon nitrides.
Conventional methods for forming the silicon oxides or silicon nitrides (used for the insulating films) are chemical vapor deposition (CVD) and thermal oxidation. The CVD method forms thin films on an insulation substrate by decomposing and reacting vapor phase compounds. In a CVD reaction chamber, the silicon oxides or silicon nitrides can be formed by supplying silicon gases into an atmosphere of oxygen or nitrogen. At this time, to obtain adhesive thin films, it is necessary to heat the substrate. Also, thermal energy, plasma energy using RF power, laser or Energy of UV light is used to decompose the reacting gas (such as silicon gas) and to accelerate the reactions of atoms or molecules decomposed by the heating of the insulation substrate and to control the physical properties of the thin films formed on the substrate.
Thermal oxidation is an "open-tube" type of reaction performed at atmospheric pressure. There is a dry process and a wet process for thermally oxidizing silicon. The dry process is the more simple thermal oxidation process. It uses only two elements, silicon and oxygen. However, it is a very slow thin film process. The wet process is performed by adding a controlled amount of water vapor to oxygen gas. Its growth rate is increased by the amount of water vapor used.
An important parameter of water vapor oxidation is the partial pressure of water vapor in the atmosphere in which the oxide film is to be formed. One atmospheric pressure is the optimal condition for the oxide film to be formed, e.g., it produces the highest rate of formation.
However, in the conventional method of forming the insulating films for the liquid crystal display where the silicon oxides (forming the gate insulating films and/or passivation layers of the thin film transistors) are formed by a CVD or thermal oxidation method, there is a need to preserve a high vacuum in the reaction chamber. Also, these processes are complicated. Further, it is a problem that these methods necessarily involve expensive devices.